A cathode ray tube (CRT) converts information contained in an input signal to electron beam energy and converts that energy into light energy to provide a visual information output on a phosphorous screen. The amount of beam modulation of the CRT and thus, the amount of light output of the CRT is a function of the voltage difference between the cathode and the first grid of the CRT (the grid closest to the cathode). If the voltage of the first grid of the CRT is held constant, then the cathode may be modulated with the voltage that represents the video information i.e. the input signal. In the event the voltage on the cathode is held constant then the voltage on the first grid may be modulated with the voltage that represents the video information i.e. the CRT input signal.
A video amplifier circuit amplifies and processes the video input signal and applies it to either the first grid or cathode of the CRT. A means of scanning the electron beam horizontally and vertically over the screen of the CRT is provided. The combination of scanning and electron beam modulation by the input signal produces an image on the screen of the CRT that will be transferred to the photographic media. The video amplifier provides a means of blanking the video signal during the horizontal and vertical retrace periods i.e. the time it takes for the electron beam to return to its starting point for scanning a horizontal line and the time it takes for the electron beam to return to its starting point for scanning the image vertically.
Photographic printers have been utilized that use a CRT as an imaging source for producing a print on a photosensitive media. A monochromatic or color CRT may be used as a imaging source and a monochromatic or color photosensitive media such as photosensitive paper or film may be used to record the CRT produced image. Typically a monochromatic image is displayed on the phosphorous screen of a CRT by modulating the electron beam of the CRT with dark to light gradations of the image and simultaneously deflecting the CRT electron beam to achieve the position of the pixels on the CRT screen. The image on the screen of the CRT may be transferred to the photographic media by any known means i.e. contact printing (the photographic media is placed directly on the CRT screen), projected by one or more lenses and/or one or more mirrors.
A monochromatic CRT may be used to expose a color picture onto photographic media. When a monochromatic display is used, three sequential exposures through red, green and blue filters will individually expose the photographic media to the red, green and blue components of the image displayed sequentially on the screen of the CRT.
A color CRT may also be used to display a color picture on a photographic media. If a color CRT was used, the color CRT would expose in one sequence the red, green and blue portions of the image on the screen of the CRT simultaneously onto the photographic media.
Assume the CRT is displaying an image from a video input signal. The illumination of the image will decrease from the center to the edges of the screen of the CRT with a gradient. The reason for the foregoing is that the electron beam has to travel an increased distance when it travels to the edge of the CRT screen. Also, the beam is less perpendicular to the phosphor screen when it is near the edges of the screen.
If the image on the screen of the CRT is transferred to the photographic media by one or more lenses, the lens or lenses will project greater illumination at the center of the photographic media than at the edges of the photographic media. The reason for the above is vignetting and cosine fourth falloff. Vignetting reduces illumination of the image for off axis image points and is related to the size and quality of the lens. Cosine fourth falloff pertains to the physics of projecting a round point of light through a lens at a off-axis angle. The projected point of light is an ellipse instead of round and its intensity is reduced compared to a round point of light resulting from an on-axis projection through the lens.
Thus, if a negative photographic media receives an image from the screen of a CRT, the density at the center of the print will be greater than the density near the edges of the print. That is, the center of the print will be darker than the edges of the print for the same video input signal amplitude. Furthermore, if a positive photographic media receives the CRT image, the center of the print will be lighter than the edges of the print for the same video input signal amplitude.